U.S. patent application number 11/937729 was filed with the patent office on 2009-05-14 for projection apparatus using solid-state light source array.
Invention is credited to Joseph R. Bietry, Robert Metzger, Barry D. Silverstein.
Application Number | 20090122272 11/937729 |
Document ID | / |
Family ID | 40289411 |
Filed Date | 2009-05-14 |
United States Patent
Application |
20090122272 |
Kind Code |
A1 |
Silverstein; Barry D. ; et
al. |
May 14, 2009 |
PROJECTION APPARATUS USING SOLID-STATE LIGHT SOURCE ARRAY
Abstract
An illumination apparatus for a digital image projector, the
illumination apparatus has a plurality of solid-state laser arrays,
each laser array with one or more rows of laser. A light combiner
has an output optical axis and a plurality of light-redirecting
prisms arranged in a stack. Each light-redirecting prism has at
least one contact surface that extends parallel to the output
optical axis and is in optical contact with an adjacent prism in
the stack and a light redirecting facet that is disposed at an
oblique angle to the at least one contact surface.
Inventors: |
Silverstein; Barry D.;
(Rochester, NY) ; Metzger; Robert; (Fairport,
NY) ; Bietry; Joseph R.; (Rochester, NY) |
Correspondence
Address: |
Frank Pincelli;Patent Legal Staff
Eastman Kodak Company, 343 State Street
Rochester
NY
14650-2201
US
|
Family ID: |
40289411 |
Appl. No.: |
11/937729 |
Filed: |
November 9, 2007 |
Current U.S.
Class: |
353/81 ; 353/20;
353/33 |
Current CPC
Class: |
G02B 19/0028 20130101;
G02B 27/0905 20130101; G02B 26/0833 20130101; H01S 5/4012 20130101;
G02B 19/0057 20130101; G02B 27/283 20130101; H01S 5/005 20130101;
G02B 27/0972 20130101; H04N 9/3161 20130101 |
Class at
Publication: |
353/81 ; 353/20;
353/33 |
International
Class: |
G03B 21/20 20060101
G03B021/20; G03B 21/14 20060101 G03B021/14; G03B 21/00 20060101
G03B021/00; G03B 21/28 20060101 G03B021/28 |
Claims
1. An illumination apparatus for a digital image projector, the
illumination apparatus comprising: a) a plurality of solid-state
laser arrays, each laser array comprising one or more rows of
lasers; and b) a light combiner having an output optical axis and
comprising a plurality of light-redirecting prisms arranged in a
stack, each light-redirecting prism comprising: (i) at least one
contact surface that extends parallel or substantially parallel to
the output optical axis and is in optical contact with an adjacent
prism in the stack; and (ii) a light redirecting facet that is
disposed at an oblique angle to the at least one contact
surface.
2. The illumination apparatus of claim 1 wherein each
light-redirecting prism has an output surface that is substantially
orthogonal to its at least one contact surface.
3. The illumination apparatus of claim 1 wherein the lasers are
vertical-cavity devices.
4. The illumination apparatus of claim 1 wherein the
light-redirecting facets provide total internal reflection for
incident light.
5. The illumination apparatus of claim 1 wherein at least one
light-redirecting facet is a coated thin film structure.
6. The illumination apparatus of claim 1 wherein at least one
light-redirecting facet is a coated metal film.
7. The illumination apparatus of claim 2 further comprising a
coating on at least one of the at least one light redirecting facet
and the output surface.
8. The illumination apparatus of claim 7 wherein the coating is
taken from the group consisting of an antireflection coating and an
IR rejection coating.
9. The illumination apparatus of claim 1 further comprising a
spatial light modulator in the path of light from the output
surface.
10. The illumination apparatus of claim 9 wherein the output
surface has an aspect ratio that is within .+-.0.3 of the aspect
ratio of the spatial light modulator.
11. The illumination apparatus of claim 2 further comprising a
waveguide for directing light from the output surface.
12. The illumination apparatus of claim 2 further comprising an
optical integrator for receiving light from the output surface.
13. The illumination apparatus of claim 11 wherein the waveguide is
an optical fiber.
14. The projection apparatus of claim 9 wherein the spatial light
modulator is taken from the group consisting of a digital
micromirror device and a liquid-crystal-on-silicon device.
15. The projection apparatus of claim 2 further comprising at least
one polarization beamsplitter in the path of the redirected light
from the output surface.
16. The projection apparatus of claim 15 further comprising a
half-wave plate in the path of light that is redirected through the
at least one polarization beamsplitter.
17. An illumination apparatus for a digital image projector, the
illumination apparatus comprising: a) a plurality of solid-state
laser arrays, each laser array comprising one or more rows of
lasers; and b) a light combiner having an output optical axis and
comprising a plurality of light-redirecting prisms arranged in a
stack, each light-redirecting prism comprising: i) at least one
contact surface that extends perpendicular or substantially
perpendicular to the output optical axis and is in optical contact
with an adjacent prism in the stack; and ii) a light redirecting
facet that is disposed at an oblique angle to the at least one
contact surface.
18. The illumination apparatus of claim 17 wherein each
light-redirecting prism has an output surface that is substantially
perpendicular to its at least one contact surface.
19. An illumination apparatus for a digital image projector, the
illumination apparatus comprising: a plurality of solid-state laser
arrays, each laser array comprising one or more rows of lasers, the
rows extending in a length direction, and each laser in the laser
arrays disposed to direct light in an emission direction that is
orthogonal to the length direction; and a light-redirecting prism
comprising a plurality of incidence facets disposed on two sides of
the light-redirecting prism and wherein the plurality of
light-redirecting facets are also disposed on the same two sides of
the light-redirecting prism.
20. The illumination apparatus as described in claim 19 wherein the
pattern of incident facets and light redirecting facets from
opposing sides forms an offset symmetry.
Description
FIELD OF THE INVENTION
[0001] This invention generally relates to an apparatus for
projecting a digital image and more particularly relates to an
improved apparatus and method using solid-state array illumination
for digital cinema projection.
BACKGROUND OF THE INVENTION
[0002] In order to be considered as suitable replacements for
conventional film projectors, digital projection systems must meet
demanding requirements for image quality. This is particularly true
for multicolor cinematic projection systems. Competitive digital
projection alternatives to conventional cinematic-quality
projectors must meet high standards of performance, providing high
resolution, wide color gamut, high brightness, and frame-sequential
contrast ratios exceeding 1,000:1.
[0003] The most promising solutions for multicolor digital cinema
projection employ, as image forming devices, one of two basic types
of spatial light modulators (SLMs). The first type of spatial light
modulator is the Digital Light Processor (DLP), a digital
micromirror device (DMD), developed by Texas Instruments, Inc.,
Dallas, Tex. DLP devices are described in a number of patents, for
example U.S. Pat. No. 4,441,791; No. 5,535,047; No. 5,600,383; and
U.S. Pat. No. 5,719,695. Optical designs for projection apparatus
employing DLPs are disclosed in U.S. Pat. Nos. 5,914,818;
5,930,050; 6,008,951; and 6,089,717. DLPs have been successfully
employed in digital projection systems.
[0004] FIG. 1 shows a simplified block diagram of a projector
apparatus 10 that uses DLP spatial light modulators. A light source
12 provides polychromatic light into a prism assembly 14, such as a
Philips prism, for example. Prism assembly 14 splits the
polychromatic light into red, green, and blue component bands and
directs each band to the corresponding spatial light modulator 20r,
20g, or 20b. Prism assembly 14 then recombines the modulated light
from each SLM 20r, 20g, and 20b and provides this light to a
projection lens 30 for projection onto a display screen or other
suitable surface.
[0005] Although DLP-based projectors demonstrate capability to
provide the necessary light throughput, contrast ratio, and color
gamut for most projection applications from desktop to large
cinema, there are inherent resolution limitations, with current
devices providing only 2148.times.1080 pixels. In addition, high
component and system costs have limited the suitability of DLP
designs for higher-quality digital cinema projection. Moreover, the
cost, size, weight, and complexity of the Philips or other suitable
prisms as well as the fast projection lens with a long working
distance required for brightness are inherent constraints with
negative impact on acceptability and usability of these
devices.
[0006] The second type of spatial light modulator used for digital
projection is the LCD (Liquid Crystal Device). The LCD forms an
image as an array of pixels by selectively modulating the
polarization state of incident light for each corresponding pixel.
LCDs appear to have advantages as spatial light modulators for
high-quality digital cinema projection systems. These advantages
include relatively large device size, favorable device yields and
the ability to fabricate higher resolution devices, for example
4096.times.2160 resolution devices by Sony and JVC Corporations.
Among examples of electronic projection apparatus that utilize LCD
spatial light modulators are those disclosed in U.S. Pat. No.
5,808,795; U.S. Pat. No. 5,798,819; U.S. Pat. No. 5,918,961; U.S.
Pat. No. 6,010,121; and U.S. Pat. No. 6,062,694. LCOS (Liquid
Crystal On Silicon) devices are thought to be particularly
promising for large-scale image projection. However, LCD components
have difficulty maintaining the high quality demands of digital
cinema, particularly with regard to color and contrast, as the high
thermal load of high brightness projection affects the materials
polarization qualities.
[0007] A continuing problem with illumination efficiency relates to
etendue or, similarly, the Lagrange invariant. As is well known in
the optical arts, etendue relates to the amount of light that can
be handled by an optical system. Potentially, the larger the
etendue, the brighter the image. Numerically, etendue is
proportional to the product of two characteristics, namely the
image area and the numerical aperture. In terms of the simplified
optical system represented in FIG. 2 having light source 12, optics
18, and a spatial light modulator 20, etendue is a factor of the
area of the light source A1 and its output angle .theta.1 and is
equal to the area of the modulator A2 and its acceptance angle
.theta.2. For increased brightness, it is desirable to provide as
much light as possible from the area of light source 12. As a
general principle, the optical design is advantaged when the
etendue at the light source is most closely matched by the etendue
at the modulator.
[0008] Increasing the numerical aperture, for example, increases
etendue so that the optical system captures more light. Similarly,
increasing the source image size, so that light originates over a
larger area, increases etendue. In order to utilize an increased
etendue on the illumination side, the etendue must be greater than
or equal to that of the illumination source. Typically, however,
the larger the image, the more costly and sizeable the optics and
support components. This is especially true of devices such as LCOS
and DLP components, where the silicon substrate and defect
potential increase with size. As a general rule, increased etendue
results in a more complex and costly optical design. Using an
approach such as that outlined in U.S. Pat. No. 5,907,437 for
example, lens components in the optical system must be designed for
large etendue. The source image area for the light that must be
converged through system optics is the sum of the combined areas of
the spatial light modulators in red, green, and blue light paths;
notably, this is three times the area of the final multicolor image
formed. That is, for the configuration disclosed in U.S. Pat. No.
5,907,437, optical components handle a sizable image area,
therefore a high etendue, since red, green, and blue color paths
are separate and must be optically converged. Moreover, although a
configuration such as that disclosed in U.S. Pat. No. 5,907,437
handles light from three times the area of the final multicolor
image formed, this configuration does not afford any benefit of
increased brightness, since each color path contains only one-third
of the total light level.
[0009] Efficiency improves when the etendue of the light source is
well-matched to the etendue of the spatial light modulator. Poorly
matched etendue means that the optical system is either
light-starved, unable to provide sufficient light to the spatial
light modulators, or inefficient, effectively discarding a
substantial portion of the light that is generated for
modulation.
[0010] The goal of providing sufficient brightness for digital
cinema applications at an acceptable system cost has eluded
designers of both LCD and DLP systems. LCD-based systems have been
compromised by the requirement for polarized light, reducing
efficiency and increasing etendue, even where polarization recovery
techniques are used. DLP device designs, not requiring polarized
light, have proven to be somewhat more efficient, but still require
expensive, short lived lamps and costly optical engines, making
them too expensive to compete against conventional cinema
projection equipment.
[0011] In order to compete with conventional high-end film-based
projection systems and provide what has been termed electronic or
digital cinema, digital projectors must be capable of achieving
comparable cinema brightness levels to this earlier equipment. As
some idea of scale, the typical theatre requires on the order of
10,000 lumens projected onto screen sizes on the order of 40 feet
in diagonal. The range of screens requires anywhere from 5,000
lumens to upwards of 40,000 lumens. In addition to this demanding
brightness requirement, these projectors must also deliver high
resolution (2048.times.1080 pixels) and provide around 2000:1
contrast and a wide color gamut.
[0012] Some digital cinema projector designs have proved to be
capable of this level of performance. However, high equipment and
operational costs have been obstacles. Projection apparatus that
meet these requirements typically cost in excess of $50,000 each
and utilize high wattage Xenon arc lamps that need replacement at
intervals between 500-2000 hours, with typical replacement cost
often exceeding $1000. The large etendue of the Xenon lamp has
considerable impact on cost and complexity, since it necessitates
relatively fast optics to collect and project light from these
sources.
[0013] One drawback common to both DLP and LCOS LCD spatial light
modulators (SLM) has been their limited ability to use solid-state
light sources, particularly laser sources. Although they are
advantaged over other types of light sources with regard to
relative spectral purity and potentially high brightness levels,
solid-state light sources require different approaches in order to
use these advantages effectively. Conventional methods and devices
for conditioning, redirecting, and combining light from color
sources, used with earlier digital projector designs, can constrain
how well laser array light sources are used.
[0014] Solid-state lasers promise improvements in etendue,
longevity, and overall spectral and brightness stability but, until
recently, have not been able to deliver visible light at sufficient
levels and within the cost needed to fit the requirements for
digital cinema. In a more recent development, VCSEL (Vertical
Cavity Surface-Emitting Laser) laser arrays have been
commercialized and show some promise as potential light sources.
However, the combined light from as many as 9 individual arrays is
needed in order to provide the necessary brightness for each
color.
[0015] Examples of projection apparatus using laser arrays include
the following:
[0016] U.S. Pat. No. 5,704,700 describes the use of a microlaser
array for projector illumination;
[0017] Commonly assigned U.S. Pat. No. 6,950,454 describes the use
of organic lasers for providing laser illumination to a spatial
light modulator;
[0018] U.S. Patent Application Publication No. 2006/0023173
describes the use of arrays of extended cavity surface-emitting
semiconductor lasers for illumination; and
[0019] U.S. Pat. No. 7,052,145 describes different display
embodiments that employ arrays of microlasers for projector
illumination.
[0020] U.S. Pat. No. 6,240,116 discusses the packaging of
conventional laser bar- and edge-emitting diodes with high cooling
efficiency and describes using lenses combined with reflectors to
reduce the divergence-size product (etendue) of a 2 dimensional
array by eliminating or reducing the spacing between collimated
beams.
[0021] There are difficulties with each of these types of
solutions. Kappel '700 teaches the use of a monolithic array of
coherent lasers for use as the light source in image projection,
whereby the number of lasers is selected to match the power
requirements of the lumen output of the projector. In a high lumen
projector, however, this approach presents a number of
difficulties. Manufacturing yields drop as the number of devices
increases and heat problems can be significant with larger scale
arrays. Coherence can also create problems for monolithic designs.
Coherence of the laser sources typically causes artifacts such as
optical interference and speckle. It is, therefore, preferable to
use an array of lasers where coherence, spatial and temporal
coherence is weak or broken. While a spectral coherence is desired
from the standpoint of improved color gamut, a small amount of
broadening of the spectrum is also desirable for removing the
sensitivity to interference and speckle and also lessens the
effects of color shift of a single spectral source. This shift
could occur, for example, in a three color projection system that
has separate red, green and blue laser sources. If all lasers in
the single color arrays are tied together and of a narrow
wavelength and a shift occurs in the operating wavelength, the
white point and color of the entire projector may fall out of
specification. On the other hand, where the array is averaged with
small variations in the wavelengths, the sensitivity to single
color shifts in the overall output is greatly reduced. While
components may be added to the system to help break this coherence
as discussed by Kappel, it is preferred from a cost and simplicity
standpoint to utilize slightly varying devices from differing
manufactured lots to form a substantially incoherent laser source.
Additionally reducing the spatial and temporal coherence at the
source is preferred, as most means of reducing this incoherence
beyond the source utilizes components such as diffusers, which
increase the effective extent of the source (etendue), cause
additional light loss, and add expense to the system. Maintaining
the small etendue of the lasers enable a simplification of the
optical train, which is highly desired.
[0022] Laser arrays of particular interest for projection
applications are various types of VCSEL (Vertical Cavity
Surface-Emitting Laser) arrays, including VECSEL (Vertical Extended
Cavity Surface-Emitting Laser) and NECSEL (Novalux Extended Cavity
Surface-Emitting Laser) devices from Novalux, Sunnyvale, Calif.
However, conventional solutions using these devices are prone to a
number of problems. One limitation relates to device yields. Due
largely to heat and packaging problems for critical components, the
commercialized VECSEL array is extended in length, but limited in
height; typically, a VECSEL array has only two rows of emitting
components. The use of more than two rows tends to dramatically
increase yield difficulties. This practical limitation would make
it difficult to provide a VECSEL illumination system for projection
apparatus as described in the Glenn '145 disclosure, for example.
Brightness would be constrained when using the projection solutions
proposed in the Mooradian et al. '3173 disclosure. Although
Kruschwitz et al '454 and others describe the use of laser arrays
using organic VCSELs, these organic lasers have not yet been
successfully commercialized. In addition to these problems,
conventional VECSEL designs are prone to difficulties with power
connection and heat sinking. These lasers are of high power; for
example, a single row laser device, frequency doubled into a two
row device from Novalux produces over 3 W of usable light. Thus,
there can be significant current requirements and heat load from
the unused current. Lifetime and beam quality is highly dependent
upon stable temperature maintenance.
[0023] Coupling of the laser sources to the projection system
presents another difficulty that is not adequately addressed using
conventional approaches. For example, using Novalux NESEL lasers,
approximately nine 2 row by 24 laser arrays are required for each
color in order to approximate the 10,000 lumen requirement of most
theatres. It is desirable to separate these sources, as well as the
electronic delivery and connection and the associated heat from the
main thermally sensitive optical system to allow optimal
performance of the projection engine. Other laser sources are
possible, such as conventional edge emitting laser diodes. However,
these are more difficult to package in array form and traditionally
have a shorter lifetime at higher brightness levels.
[0024] None of the solutions yet proposed have addressed the
problem of etendue-matching of the laser sources to the system,
thermally separating the illumination sources from the optical
engine. Nor have these solutions adequately addressed the need to
use polarized light from the laser devices more effectively.
[0025] Thus, it can be seen that there is a need for illumination
solutions that capitalize on the advantages of solid-state array
light sources and allow effective use of solid-state illumination
components with DLP and LCOS modulators.
SUMMARY OF THE INVENTION
[0026] It is an object of the present invention to address the need
for improved illumination apparatus used with digital spatial light
modulators such as DLP and LCOS and related microdisplay spatial
light modulator devices. With this object in mind, the present
invention provides an illumination apparatus for a digital image
projector, the illumination apparatus comprising: a plurality of
solid-state laser arrays, each laser array comprising one or more
rows of lasers; and a light combiner having an output optical axis
and comprising a plurality of light-redirecting prisms arranged in
a stack, each light-redirecting prism comprising: at least one
contact surface that extends parallel or substantially parallel to
the output optical axis and is in optical contact with an adjacent
prism in the stack; and a light redirecting facet that is disposed
at an oblique angle to the at least one contact surface.
[0027] It is a feature of the present invention that it provides
ways to improve etendue matching between illumination and
modulation components.
[0028] These and other objects, features, and advantages of the
present invention will become apparent to those skilled in the art
upon a reading of the following detailed description when taken in
conjunction with the drawings wherein there is shown and described
an illustrative embodiment of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] While the specification concludes with claims particularly
pointing out and distinctly claiming the subject matter of the
present invention, it is believed that the invention will be better
understood from the following description when taken in conjunction
with the accompanying drawings, wherein:
[0030] FIG. 1 is a schematic block diagram of a conventional
projection apparatus using a combining prism for the different
color light paths;
[0031] FIG. 2 is a representative diagram showing etendue for an
optical system;
[0032] FIGS. 3A, 3B, and 3C are plan views showing the relative
fill factor of different solid-state light array-to-light guide
combinations;
[0033] FIG. 4A is a schematic side-view diagram showing one method
for combining light from multiple solid-state light arrays along
the same illumination path;
[0034] FIG. 4B is a schematic side-view diagram showing an
alternate method for combining light from multiple solid-state
light arrays along the same illumination path;
[0035] FIG. 5 is a perspective view of the configuration for
combining light shown in FIG. 4A;
[0036] FIG. 6 is a schematic block diagram showing the general
arrangement of a projection apparatus using the illumination
combiner of the present invention;
[0037] FIG. 7A is a schematic side-view diagram showing the use of
a light-redirecting prism for combining illumination from multiple
solid-state light arrays in one embodiment;
[0038] FIG. 7B is a perspective view showing the configuration of
FIG. 7A;
[0039] FIG. 7C is a perspective exploded view of a segmented
illumination combiner according to one embodiment;
[0040] FIG. 8 is a schematic side-view diagram showing the use of a
light-redirecting prism for combining illumination from multiple
solid-state light arrays in another embodiment;
[0041] FIG. 9 is a schematic side-view showing the use of an offset
symmetric embodiment of a light-redirecting prism that accepts
light from both sides;
[0042] FIG. 10 is a side view showing light-redirecting prisms
arranged to form an offset symmetric illumination combiner;
[0043] FIG. 11 shows calculations that can be applied for obtaining
parallel light output from a light-redirecting prism;
[0044] FIG. 12 shows aspect ratio comparisons for the output face
of a light redirecting prism and the spatial light modulator in one
embodiment; and
[0045] FIG. 13 shows aspect ratio matching using polarized
illumination.
DETAILED DESCRIPTION OF THE INVENTION
[0046] Figures shown and described herein are provided to
illustrate principles of operation according to the present
invention and are not drawn with intent to show actual size or
scale. Because of the relative dimensions of the component parts
for the laser array of the present invention, some exaggeration is
necessary in order to emphasize basic structure, shape, and
principles of operation.
[0047] The term "oblique" as used in the present disclosure has its
conventional meaning, in which an angular relationship to a
reference line or plane is not parallel or other integer multiple
of 90 degrees. An oblique angle is thus either greater than or less
than a right (90 degree) angle and is not parallel with respect to
its reference.
[0048] Embodiments of the present invention address the need for
improved brightness using solid-state arrays and provide solutions
that can allow ease of removal and modular replacement of laser
assemblies. Embodiments of the present invention also provide
features that reduce thermal effects that might otherwise cause
thermally induced stress birefringence in optical components that
are used with LCOS projectors.
[0049] One approach used to reduce thermal loading by embodiments
of the present invention is to isolate the light sources from light
modulation components using a waveguide structure. Light from
multiple solid-state light source arrays is coupled into optical
waveguides that deliver the light to the modulation device.
Moreover, the geometry of the light source-to-waveguide interface
can be optimized so that the waveguide output is well matched to
the aspect ratio of the spatial light modulator. In practice, this
means that the waveguide aperture is substantially filled or
slightly underfilled for maintaining optimal etendue levels. This
arrangement also helps to minimize the speed requirement of
illumination optics. Referring to FIGS. 3A, 3B, and 3C, the input
aperture of a light guide 52 is shown in cross section. A
solid-state light array 44 is shown as it would appear at the input
aperture of light guide 52, properly scaled. As shown in FIG. 3A,
the aperture is underfilled, which may easily cause a poor etendue
match at the spatial light modulator end of light guide 52. In FIG.
3B, the aspect ratios of array 44 and light guide 52 are well
matched by reshaping the input aperture of light guide 52 from its
conventional circular form. FIG. 3C shows another arrangement in
which multiple arrays 44' are combined with array 44 to effectively
form a larger array. Methods of combining multiple arrays 44 are
described subsequently.
[0050] In embodiments using this approach, an optical fiber can be
utilized for light guide 52. In one embodiment, a rectangular core
optical fiber is used. For example, rectangular core fiber from
Liekki of Lohaja, Finland has been fabricated to better match
source aspect ratios. In this case, techniques such as taught by
U.S. Pat. No. 6,240,116 to Lang et al. where stepped mirrors can be
used, can shape the light from multiple arrays 44 to form a
rectangular aspect ratio source with a smaller etendue. The
approach shown in the Lang et al. '116 disclosure uses discrete
diodes, whereby the vertical cavity lasers used in the preferred
embodiment inherently have a low divergence angle and therefore do
not require the use of a lens to collimate the beam. In an
alternative approach, as shown in FIG. 4A and in perspective view
in FIG. 5, one or more interspersed mirrors 46 may be used to place
the optical axis of additional arrays 44' in line with array 44 to
provide the arrangement shown in cross-section in FIG. 3C. A more
direct example using combined arrays 44 is shown in FIG. 4B.
However, it can be appreciated that heat and spacing requirements
may limit how many arrays 44 can be stacked in this manner.
[0051] The schematic diagram of FIG. 6 shows a basic arrangement
for projection apparatus 10 that is used in a number of embodiments
of the present invention. Three light modulation assemblies 40r,
40g, and 40b are shown, each modulating one of the primary Red,
Green, or Blue (RGB) colors from an illumination combiner apparatus
42. In each light modulation assembly 40r, 40g, and 40b, an
optional lens 50 directs light into light guide 52, such as an
optical fiber. At the output of light guide 52, a lens 54 directs
light through an integrator 51, such as a fly's eye integrator or
integrating bar, for example, to a spatial light modulator 60,
which may be a DLP, LCOS, or other light modulating component.
Projection optics 70, indicated generally in a dashed outline in
FIG. 6 due to many possible embodiments, then direct the modulated
light to a display surface 80. The basic arrangement shown in FIG.
6 is used for subsequent embodiments of the present invention, with
various arrangements used for illumination combiner apparatus
42.
[0052] One problem of particular concern for large-scale projectors
relates to the high brightness requirements and concomitant heat
load that must be handled by the illumination optics. In order to
take advantage of the low etendue, a typical digital cinema
projector requiring 10,000 lumens would concentrate around 24 watts
of power within approximately 4 square centimeters. When using
solid-state laser arrays as the light source, the high heat levels
such as these that may result can have significant effect on
illumination combiner apparatus 42. Molded plastic assemblies,
suitable for low-temperature operation, are limited to certain
threshold heat levels; beyond these levels, birefringence, material
damage, and other negative effects can result.
[0053] The limitations of optical plastics are a limitation even
for high-end polymers having relatively high thermal
characteristics. For example, one of the most durable optical
plastics designed for molding is Zeonex, a cyclo-olefin polymer
manufactured by Zeon Corporation, Louisville, Ky. This material has
been shown to absorb about 2% of blue wavelength light between 420
nm and 450 nm, within the spectrum required to achieve appropriate
color gamut for digital cinema applications. Additionally, this
absorption increases by around 1% with as little a 2500 hours at
room temperature and at a relatively small energy density of 400 mW
per square centimeter. Yellowing can occur with this absorption
level. As this example illustrates, molded plastics would be
impractical for high-lumen illumination applications using
solid-state laser arrays.
[0054] A molded glass assembly would be a possible alternative as a
light combiner for high-lumen illumination applications. However,
even when using high-temperature glasses and state-of-the-art glass
molding technologies, fabrication of a suitable combiner device as
a block of glass, considering the high heat stresses that are
likely, may not be feasible.
[0055] Typical quality molded glass components range from aspheric
lenses to lenslet arrays. Historically, Eastman Kodak Company,
Rochester N.Y. has fabricated molded aspheric lenses up to 2''
diameter with relatively thin center thicknesses. Companies such as
Docter Optics, Germany and Izuzu Glass, Japan have molded plates up
to 2'' diagonals. In both cases the process begins with a glass
perform that is heated and pressed between two surfaces. Since the
materials do not start in a molten form, it is difficult to achieve
even heating throughout, particularly with a thick component such
as a prism. The need to heat the entire component can be minimized
by using a more featured preform, whereby only the critical optical
surfaces are molded. This technique, however, creates a potentially
damaging differential stress between the outside of the prism and
the inside. This stress can easily deteriorate the polarization
properties of component. Therefore, molded glass prisms of this
form are difficult to fabricate and do not perform as well as
conventional bulk glass components.
[0056] The task of glass molding is constrained by the fact that
only certain glass types have been found to mold effectively and in
the difficulty of molding flats into glass. Glasses such as B270,
for example, are commonly utilized for molding in order to achieve
the consistent molding surface properties required by optical
components. But other types of glass cannot be readily molded. This
further limits the capability of forming a glass combiner that
meets both the requirements of the molding process and the
requirements of handling high light levels without compromising
optical properties of the laser light.
[0057] For embodiments of the present invention, illumination
combiner apparatus 42, used as a light redirector or light
combiner, has a composite prism structure. Illumination combiner
apparatus 42 is formed in segmented fashion from multiple prisms
that are stacked together, each in optical contact with its
neighbor along at least one surface. Components of illumination
apparatus 42 include the light sources, provided by solid-state
laser arrays, and the light combiner that is provided by a
composite prism structure.
[0058] FIGS. 7A and 7B show side and orthogonal views,
respectively, of an embodiment of illumination combiner apparatus
42 as an assembly with a composite light-redirecting prism 30
formed in this manner that combines laser light from four
solid-state light arrays 44 . Since this combiner is fabricated
from segmented glass pieces that are made in a conventional grind
and polish method, there are no restrictions on the glass type that
may be used. Therefore materials difficult to mold, such as fused
silica, for example, can be used. Fused silica exhibits very little
absorption, thus exhibiting negligible substrate heat up during
operation. Alternatively, glass materials with a low coefficient of
stress birefringence may be used, such as SF57, that only minimally
create retardation upon either mechanical or thermal stress. Most
of these latter type glasses are lead based and can be quite
difficult to handle, therefore materials such as fused silica,
provided with suitable coatings and low-absorption adhesives and
using low-stress mounting with stable thermal environments, are
advantaged for building this assembly. Formed from these materials,
a segmented laser array combiner can handle optical densities of at
least up to 6 W/cm.sup.2 without significant degradation to the
optical properties of light from the laser sources, which is well
beyond the capability of a molded part.
[0059] Composite light-redirecting prism 30 has at least one
incidence facet 32 that accepts light emitted from array 44 in an
emission direction D1. Light is redirected to an output direction
D2 that is parallel to output axis O and can be generally
orthogonal to emission direction D1. Light redirecting prism 30,
formed as described herein, has a plural number of
light-redirecting facets 38. Each light-redirecting facet 38 is at
an oblique angle relative to emission direction D1 and provides
redirection, using a reflective coating or Total Internal
Reflection (TIR), to incident light that is emitted from lasers 26.
For example, light-redirecting facets 38 could be coated thin film
structures or coated metal film. When staggered as shown in FIGS.
7A and 7B, these features help to narrow the light path for this
illumination. As FIG. 7B shows, light arrays 44 have multiple
lasers 26 that extend in a length direction L. Light-redirecting
facets 38 and other facets also extend in direction L. The emission
direction D1 for each of light arrays 44 is orthogonal (that is,
perpendicular) to the length direction L. An output surface 34 then
provides redirected light from the light arrays 44.
[0060] The perspective exploded view of FIG. 7C shows the
components of light redirecting prism 30 as a composite prism
structure in one embodiment. Composite light-redirecting prism 30
is formed as a stack of light-redirecting prisms 72. Each prism 72
has light-redirecting facet 38 for redirecting incident light in
the direction of the optical axis O. Each prism 72 also has at
least one contact surface 76 that is parallel to optical axis O
and, in the assembled composite light-redirecting prism 30, is
disposed in optical contact with an adjacent prism 72. Optical
contact between neighboring prisms 72 can be effected using an
optical adhesive or other intermediary material having a suitable
index of refraction, for example, or by applying a holding pressure
to the stack of prisms 72.
[0061] The cross-sectional side view of FIG. 8 shows an alternate
embodiment using a stack of prisms 72, again with a single
incidence facet 32 for light-redirecting prism 30 in illumination
apparatus 42, in which light-redirecting facets 38 of light
redirecting prism 30 are scaled so that each light-redirecting
facet 38 redirects light from multiple rows of lasers 26 at a
time.
[0062] The cross-sectional side view of FIG. 9 shows another
embodiment of illumination combiner apparatus 42 with
light-redirecting prism 30 that provides even more compact
arrangement of illumination components using solid-state arrays.
Light redirecting prism 30 has two sides, each facing one or more
solid-state light arrays 44. Light-redirecting facets 38 are
staggered or offset so that light incident on the opposite side of
light redirecting prism 30 is directed to an appropriate
light-redirecting facet 38. In this arrangement, each side of
composite light redirecting prism 30 has a plural number of
incidence facets 32 and a plural number of light-redirecting facets
38. This allows light-redirecting prism 30 to accept light from
arrays 44 that face each other, with generally opposing emission
directions D1 and D1'. As noted with respect to FIG. 7B, the
different emission directions D1 and D1' for each of the respective
light arrays 44 are both orthogonal with respect to the length
direction for array 44 rows. Each side of composite prism 30, then,
has these two types of facets: light-redirecting facets 38 and
incidence facets 32 that can be normal or at a near-normal angle
with respect to the incident light from the corresponding array 44.
Light redirecting facets 38 on each side are in parallel. The paths
of light from opposite sides of light redirecting prism 30 are
interleaved within prism 30.
[0063] The side view of FIG. 10 shows an embodiment of composite
light-redirecting prism 30 using a stack of prisms 72 that are
disposed together with surfaces 76 in optical contact, wherein each
prism 72 has the same dimensions. Here, since each prism 72 has an
output surface 78, the effective output surface of composite
light-redirecting prism 30 has multiple facets. Optionally, an
assembled stack of prisms 72 can be cut at a line K so that the
combined prisms 72 provide composite light-redirecting prism 30
with a planar output surface. While this embodiment shows the
segments cut parallel to the output beams, it is possible to also
fabricate segments that are substantially perpendicular to the
light output beams.
[0064] Segmented prism assemblies that combine and compact the
light from laser arrays can be designed in many shapes and forms.
In many instances, their compactness and symmetry are beneficial to
the ease of alignment, design and fabrication simplicity, thus
leading to a lower cost solution. FIG. 11 shows light handling
considerations for light redirecting prism 30 in illumination
combiner apparatus 42 for an application in which light enters from
two sides in a symmetric fashion. Light redirecting facets 38 are
offset to allow light incident on an opposite side of light
redirecting prism 30 to be redirected. Using the reference
coordinate system of FIG. 11, the lasers on opposite sides of prism
30 are offset in the vertical or Y direction. Similarly, the prism
assemblies are also offset in this vertical direction by a
corresponding amount. This offset symmetry enables left and right
sided parts to be fabricated with either identical or mirrored
assemblies and tooling. Likewise, the laser holders or mounts may
be reused with identical or mirror parts, thus reducing setup times
for the fabrication or assembly processes. This is particularly
valuable in the process steps required in grinding and polishing of
the optical facets.
[0065] Normal angular orientation allows for easier alignment of
the various laser modules to composite light-redirecting prism 30
by retro-reflection of a small residual light from an
anti-reflection coated face back into each of the lasers. This
retro-reflection can be useful as a means of creating a subtle
external cavity that may induce mode instability in the laser.
While such mode hopping may be considered noise under typical
applications, this noise can add value in projection applications
by further reducing the laser coherence (and thus reducing
inter-laser coherence) thereby reducing visual speckle at the image
plane. Additionally, with this dual sided approach, laser modules
are interleaved with light from different neighboring modules,
providing a source of further spatial mixing when optically
integrated further in the optical system. This again helps to
reduce possible speckle and increase system uniformity.
[0066] While it can be seen that this normal orientation of
incidence facets 32 of prism 30 to laser 44 is preferred, normal
incidence light with respect to the incidence facets 32 or output
surface 34 is not required for combining the illumination sources.
It is required, however, that the light paths for light exiting
prism 30 at surface(s) 34 be substantially parallel to each other.
Obtaining parallel paths requires control of a number of factors,
such as the following:
[0067] (i) the combination of the angle of incidence of the lasers
44 on each side (as they may be different) to input facets on each
side;
[0068] (ii) the refraction in the prism 72 segments based on the
index of refraction of the material;
[0069] (iii) the reflection from light redirecting facets 38 from
each side (again these may be different on each side); and
[0070] (iv) the refraction for light exiting of prism 30.
[0071] These factors must cooperate so that output light paths from
the exit face(s) are parallel.
[0072] FIG. 11 shows geometrical considerations for providing
parallel output paths, with light parallel to output direction D2,
when composite light redirecting prism 30 accepts light from two
sides, as shown in the general arrangement of FIG. 9. In the
particular example of FIG. 9, a simple embodiment is shown, with
the incident light perpendicular at each incidence facet 32. FIG.
11 deals with the complexity of the more general case, in which
incident light is at some oblique angle with respect to incidence
facet 32. Refractive index n.sub.1 or n.sub.1' is air or other
surrounding medium. Refractive index n.sub.2 or n.sub.2' is that of
light redirecting prism 30.
[0073] Ray origin L is at left; ray origin R at right. A reference
x-y coordinate axis is shown at these origins, at surfaces of
incidence, and at surfaces at which light is reflected. Light from
ray origin L is at an angle .theta..sub.1 relative to the reference
coordinate system. Angle .phi..sub.1 is the normal to the incident
surface of incidence facet 32. Refraction at facet 32 directs this
light at angle .theta..sub.2 relative to the given x-axis. This
light then reflects from redirection surface 38 that has a surface
normal of .sigma..sub.1. With respect to the x-axis, output
direction D2 is at an angle .beta..sub.1 from the x-axis where:
.beta..sub.1=180-2.sigma..sub.1-.theta..sub.2 (eq. 1)
[0074] The optical path from the right is similar. Light from ray
origin R is at an angle .theta..sub.1' relative to the reference
coordinate system. Angle .phi..sub.1' is normal to incidence facet
32. Refraction at facet 32 directs this light at angle
.theta..sub.2' relative to the given x-axis. This light then
reflects from the surface that has a surface normal of
.sigma..sub.1'. With respect to the x-axis, output direction D2 is
at an angle .beta..sub.1' from the x-axis where:
.beta.'.sub.1=180-2.sigma.'.sub.1-.theta.'.sub.2 (eq. 2)
From FIG. 10 it can be seen that
.beta..sub.1+.beta.'.sub.1=180 (eq. 3)
.beta..sub.1=180-.beta.'.sub.1 (eq. 4)
So that
180-2.sigma..sub.1-.theta..sub.2=180-(180-2.sigma.'.sub.1-.theta.'.sub.2-
) (eq. 5)
180-2.sigma..sub.1-.theta..sub.2=2.sigma.'.sub.1+.theta.'.sub.2
(eq. 6)
Applying Snell's law for refracted light originating at ray origin
L and incident from the left, using the notation for FIG. 10:
n 1 sin ( .phi. 1 - .theta. 1 ) = n 2 sin ( .phi. 1 - .theta. 2 ) (
eq . 7 ) sin ( .phi. 1 - .theta. 2 ) = n 1 n 2 sin ( .phi. 1 -
.theta. 1 ) ( eq . 8 ) ( .phi. 1 - .theta. 2 ) = sin - 1 ( n 1 n 2
sin ( .phi. 1 - .theta. 1 ) ) ( eq . 9 ) .theta. 2 = .phi. 1 - sin
- 1 ( n 1 n 2 sin ( .phi. 1 - .theta. 1 ) ) ( eq . 10 )
##EQU00001##
Assuming that n.sub.2=n.sub.2':
.theta. 2 ' = .phi. 1 ' - sin - 1 ( n 1 n 2 sin ( .phi. 1 ' -
.theta. 1 ' ) ) ( eq . 11 ) ##EQU00002##
Substituting from eq. 6:
180 - 2 .sigma. 1 - ( .phi. 1 - sin - 1 ( n 1 n 2 sin ( .phi. 1 -
.theta. 1 ) ) ) = 2 .sigma. 1 ' + ( .phi. 1 ' - sin - 1 ( n 1 n 2
sin ( .phi. 1 ' - .theta. 1 ' ) ) ) ( eq . 12 ) ##EQU00003##
[0075] Where eq. 12 is satisfied for the corresponding materials
and angles, the output light that is redirected from light
redirecting prism 30 will be in parallel for incident light from
each side.
[0076] In a projector embodiment, such as shown in the schematic
block diagram of FIG. 6, it is useful to substantially match the
width:height aspect ratio of output surface 34 to the aspect ratio
of its corresponding spatial light modulator 60. (As overfilling of
the spatial light modulator is required, identical overfilling will
result in a slight difference in aspect ratio.) This relationship
is shown in FIG. 12 as w:h. As a rule-of-thumb, the w:h aspect
ratio for output surface 34 should be within about .+-.0.3 of the
aspect ratio for spatial light modulator 60. Light guide 52 (FIG.
6) or other waveguide element may also match the aspect ratio,
while scaled to half size or other scaled dimensions.
[0077] Composite light directing prism 30 can be made from various
types of highly transmissive materials for typical desktop and
business projection applications. For these relatively low power
applications, plastics may be chosen. It is preferred to use
fabrication processes that induce very little stress to the part,
particularly with plastics, to reduce birefringence. Similarly, it
is desirable to choose materials that induce minimal stress or
thermally induced birefringence. Plastics such as acrylic or Zeonex
from Zeon Chemicals would be examples of suitable materials. This
can be particularly important where the combiner is utilized in a
polarization based optical system.
[0078] For higher power applications, such as for digital cinema
projection where many high power lasers are required, plastics are
impractical. Heat buildup from even small level of optical
absorption could ultimately damage the material and degrade the
light transmission. The light absorption in the material will also
induce further stress birefringence that degrades the laser
polarization states. For high lumen systems low absorption glass is
preferred Glasses such as fused silica would absorb minimal light,
thereby maintaining a stable temperature. This would prevent
thermally induced stress birefringence from degrading the
polarization states.
[0079] Alternatively to low absorption glasses, minimizing stress
birefringence can be achieved by using a glass having low stress
coefficients of birefringence, such as SF57. Where molding is
desired, a slow mold process would be preferred, with annealing to
reduce any inherent stress. A clean up polarizer may be desired or
necessary to remove any rotated polarization states that might
develop from residual birefringence. In general, however, these
types of materials may not be conducive to creating such a molded
glass component, thus requiring a more conventional polishing
technique. This can be fabricated from a single piece of material
or preferably an assembly of multiple segments to make up the
completed prism as described earlier. The individual segments can
be fabricated in plate form, with the sides polished for the input
face and the end faces of the plates polished to the correct angles
required. This method both simplifies the fabrication process and
maintains the low stress required by high lumen systems.
[0080] As mentioned earlier, the segments of this combining prism
could be non-identical. For example, if the segments are made with
cuts parallel to the output face, the top most segment would be
thinner than the bottom most segment. This approach may be
advantaged from the standpoint of not fabricating pates that are
long and thin, that may be difficult to assemble. In either case,
utilizing segments, allow a conventional grind and polish operation
to fabricate simple inexpensive parts that are assembled into a
relatively complex, yet highly thermally stable prism combiner.
[0081] A number of improvements could be made to the basic prism 30
design shown in FIGS. 7A-9. For example, sides and output surface
34 of prism 30 could be antireflection coated or coated with
spectrally rejecting thin film. Many of the lasers, such as the
NESCEL's discussed earlier have residual light, often infrared,
that must be rejected to prevent heating issues further in the
system.
[0082] Embodiments of the present invention are useful for shaping
the aspect ratio of the light source so that it suits the aspect
ratio of the spatial light modulator that is used. Embodiments of
the present invention can be used with waveguides of different
dimensions, allowing the waveguide not only to be flexible, but
also to be shaped with substantially the same aspect ratio to that
of the modulator. For digital cinema this would be approximately
1.9:1.
[0083] An alternative embodiment could use a square core fiber. In
this case, the laser array would again be fabricated to match the
aspect ratio of the square fiber. The methods utilized in earlier
embodiments along with the optional use of polarization combining
would work similarly. On the output side, the square aspect ratio
would not appropriately match the modulator aspect ratio. In this
case, however, there might be the desire to use a polarized
illumination such as required by an LCOS spatial light modulator.
Referring to the schematic diagram of FIG. 13, there is shown an
embodiment of a light modulation assembly 40 that provides
polarized light and uses a polarization recovery technique to adapt
the illumination aspect ratio to the aspect ratio of its spatial
light modulator 60. Here, light guide 52 has an aspect ratio that
is approximately half of the aspect ratio of spatial light
modulator 60. The illumination that is output from light guide 52
and is directed through lens 54 is substantially unpolarized.
Conventional dot and arrow notation is used in FIG. 13 to designate
polarization states. A first polarization beamsplitter 62 transmits
s-polarized light and reflects p-polarized light toward a second
polarization beamsplitter 64. Polarization beamsplitter 64 reflects
the p-polarized light through a half-waveplate 66 that changes the
polarization state to s-polarized light. In this way, the
illumination that is incident on integrator 51 and goes to spatial
light modulator 60 is highly polarized. Moreover, the aspect ratio
of this illumination is increased in the width w direction, as
indicated in FIG. 13. This arrangement effectively doubles the area
of the light source to provide improved aspect ratio matching and
provides a uniform, polarized light that is particularly suitable
for LCOS devices.
[0084] Similarly, a round core optical waveguide, such as common
multimode optical fiber can be utilized. In this case, the laser
source array could be square as discussed in the prior embodiment.
To make best use of this laser array, however, the array source
must be focused down to a smaller size than that of the optical
fiber core. This means that the angle of the light entering the
waveguide is increased. Thus some etendue loss is associated with
this mismatch in shape. This mismatch may be reduced, however, if
the laser array could be rounded by the use of shorter laser arrays
with fewer active elements, (improving the effective laser device
yield by allowing usage of shorter cut arrays), combined on the
edges of the square array to "round out" the effective source.
While this would provide a higher brightness, some etendue loss
will occur on the modulator side when the output is uniformized and
matched to the rectangular shape of the modulation device.
[0085] A fiber optical waveguide, being multi-mode, will not
preserve the inherent polarization of the lasers. Therefore, a
device such as a DLP modulator can directly use the unpolarized
light, after uniformizing, mixing, or optically integrating, such
as using an integrating bar or lenslet array.
[0086] While an optical waveguide between the illumination combiner
apparatus 42 and integrator 51 is the embodiment shown for light
guide 52, other methods for relaying and separating the
illumination sources from the projection optical engine are
possible. Relaying with standard lenses would be another approach
for achieving the desired thermal and spatial separation.
[0087] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention. For example, where laser
arrays are described in the detailed embodiments, other solid-state
emissive components could be used as an alternative. Supporting
lenses and other optical components may also be added to each
optical path.
[0088] Thus, what is provided is an apparatus and method using
solid-state array illumination for digital cinema projection.
[0089] The invention has been described with reference to a
preferred embodiment. However, it will be appreciated that
variations and modifications can be effected by a person of
ordinary skill in the art without departing from the scope of the
invention.
PARTS LIST
[0090] 10. Projector apparatus [0091] 12. Light source [0092] 14.
Prism assembly [0093] 18. Optics [0094] 20, 20r, 20g, 20b. Spatial
light modulator [0095] 26. Laser [0096] 30. Light redirecting prism
[0097] 32. Incidence facet [0098] 34. Output surface [0099] 38.
Light-redirecting facet [0100] 40, 40r, 40g, 40b. Light modulation
assembly [0101] 42. Illumination combiner apparatus [0102] 44, 44'.
Solid-state light array [0103] 46. Mirror [0104] 48, 56.
Polarization beamsplitter [0105] 50. Lens [0106] 51. Integrator
[0107] 52. Light guide [0108] 54. Lens [0109] 60. Spatial light
modulator [0110] 62, 64. Polarization beamsplitter [0111] 66.
Waveplate [0112] 70. Projection optics [0113] 72. Prism [0114] 76.
Surface [0115] 78. Surface [0116] 80. Display surface [0117] D1,
D1'. Emission direction [0118] D2. Output direction [0119] L. Ray
origin [0120] R. Ray origin [0121] h. Height [0122] w. Width
* * * * *